Dendritic web crystals are grown according to known methods by withdrawing (growing) a continuous ribbon or web from a pool of molten silicon. See, for example, U.S. Pat. No. 4,389,377. Generally, the method involves placing a silicon charge in a quartz crucible which is housed in a susceptor, placing heat shields over the crucible, melting the silicon using an inductive coil heater or other known heating methods, placing a seed dendrite in the melt, undercooling to form a button and pulling the button from the melt to drop dendrites from each end of the button. The dendrites grow into the melt and on continued pulling of the button from the melt, the dendrites support a liquid film of silicon that freezes as the material passes through a slot in the shields, thereby growing a single crystal silicon ribbon (also known as silicon dendritic web), between the dendrites.
In the standard growth process, precise amounts of boron or phosphorous are added to the molten silicon so that as the ribbon is growing, boron or phosphorous atoms are included in the ribbon structure to form a p-type or n-type crystal, respectively. N-type crystals are those having an excess of electrons, while p-type crystals are those having an excess of holes. The concentration of these "impurity" atoms in the silicon ribbon determines the base resistivity of the crystal, a key parameter in the efficiency of photovoltaic cells produced from a silicon ribbon crystal.
However, the use of phosphorus as a dopant has certain drawbacks. Dendritic web crystals are generally grown according to a semi-continuous process, whereby silicon feedstock is injected into the growth furnace as the ribbon crystals are being withdrawn. Maintaining uniform resistivity in an n-type phosphorous doped web requires that the dopant be replenished continuously along with the silicon to maintain a constant dopant concentration in the melt. However, it is extremely difficult to control the addition of phosphorus to the melt in order to obtain a uniform resistivity along the entire length of a ribbon crystal which is typically five to fifteen meters long and requires 5 to 15 hours of growing time. Thus, it would be useful to develop a doping method whereby the resistivity of the crystal could be controlled to be more uniform. It would also be useful to develop a method of doping in which there would be no need to replenish the dopant added after the initial charge.
Although the inventors are aware of no publications to this effect, it is believed that antimony may have been used as a dopant in the Czochralski crystal growth process, in the growth of n-type boules. However, antimony, if used at all, is not used as a dopant for any specific reason in Czochralski growth other than in order to conduct certain tests. Normally, n-type Czochralski boules are grown with phosphorus doping.